This invention relates to a pattern forming method using contrast enhanced material, and more particularly to a pattern forming method using contrast enhanced material for forming fine patterns capable of further improving the resolution.
Improvement of large scale integration and high density integration of semiconductor integrated circuits has been intensified with the progress of conventional lithographic techniques. The minimum line width has become about 1 .mu.m or so. In order to attain the processed line width, there can be mentioned various means including an ultraviolet ray-exposing method using a wafer stepper with a large numerical aperture (large NA) lens, an electronic beam-exposing method for direct pattern-formation on a support and a proximity exposing method using an X-ray.
Of these means, the first ultraviolet ray-exposing method using a wafer stepper is the best for formation of patterns with no deterioration of throughput.
In 1983, B. F. Griffing et al. (GE Company, U.S.A.) disclosed a method for improving resolving power and pattern form by laminating a contrast-enhanced layer capable of enhancing the contrast of an optical strength profile on a resist for formation of patterns (Contrast Enhanced Photolithograph; B. E. Griffing et al., IEEE-ED, VOL. EDL-4, No. 1, Jan. 1983).
They reported in their disclosure that the resolution of up to 0.4 .mu.m was possible with a wafer stepper (.lambda.; 436 nm, N.A. ; 0.32).
As a result of the study by the present inventors, the characteristics of a pattern-forming organic film for contrast-enhancement will be explained as follows:
In general, an optical strength profile of an output in a wafer stepper is processed by the optical lens system thereof. In more detail, when an ultraviolet ray exposure is effected through a reticle, even though the diffraction-free and ideal input optical strength profile is a complete rectangular wave, the contrast C thereof is represented by the following formula: ##EQU1##
In this case, the contrast C is 100%. The input wave is, after having passed through the optical lens, subjected to Fourier's conversion in accordance with the transfer function of the optical lens system, and thereafter this becomes an output wave having a form that approximates a cosine wave and the contrast C thereof becomes deteriorated. The deterioration of the contrast has a great influence on the pattern formation, for example, including resolving power and pattern form. In this connection, the contrast required for resist pattern resolution is said to be 60% or more than the characteristic of the resist itself, and if the contrast value C becomes less than 60%, the pattern formation would become impossible.
Under the above situation, it is noted that when the above-mentioned output wave is passed through a pattern-forming organic film having such characteristic curve that the transmission to an ultraviolet ray is apt to be small in the range of a short exposure time (small exposure energy) (that is, increment of I.sub.min is small), while the transmission to an ultraviolet ray is apt to be large in the range of a large exposure energy (that is, increment of I.sub.max is large), the contrast value C is apt to increase. In order to quantitatively explain the fact in more detail, a parameter as set forth in F. H. Dill et al's report of "Characterization of Positive Photoresist" (by F. H. Dill et al., IEEE-ED, VOL, ED-22, No. 7, Jul., 1975), which is an exposure and absorption factor A of a positive resist, is used. In general, A is represented by the following formula (2), and the tendency of enlargement of the value A is desired for contrast enhancement. ##EQU2## In order to attain the tendency of enlarging the value A, it is necessary to thin the film thickness (d) and to enlarge the ratio of the initial transmission [T(.infin..sub.0)] to the final transmission [t(.infin.)].
In the conventional contrast enhanced layer containing photodegradable reagent, the coefficient A of said contrast enhancement is 10 or less. At this level of coefficient, the resolution of the resist pattern is only 0.5 .mu.m or more, and it is impossible to cope with 0.5 .mu.m in the 16-Mbit DRAM (dynamic random access memory), for example, in the memory field when the device is microminiaturized in the future.
The present inventors had proposed to use, as the contrast enhanced material, water-soluble polymer advantageous in semiconductor process, such as pullulan and polyvinylalcohol, but since they are inferior in gas permeability, it was found that pattern defects are caused when the N.sub.2 gas and other gases generated from the resist are entrapped in the contrast enhanced layer. This phenomenon is explained by referring to FIG. 3. The gas generated when exposed with light 3 of 436 nm from a positive resist 2 on a semiconductor substrate 1 is taken into a conventional contrast enhanced layer 5, and is left over in the layer 5 as bubbles 6 as shown in FIG. 3A. Such bubbles deteriorate the ground resist pattern at the time of developing, and are transferred on the resist 2, thereby causing defective resist pattern 2 as shown in FIG. 3B. At the same time, in the conventional contrast enhanced layer, the value of A is 10 or less, and the shape becomes inferior in a pattern of 0.5 .mu.m or less as in 2b. When a high performance contrast enhanced layer with a higher contrast enhanced effect is used, the formation of defective resist pattern 2b due to bubbles 6 becomes more obvious, and when the contrast enhancement effect is improved, such defects must be removed more carefully, and it is also necessary to reduce the through-put on the semiconductor process at the same time.